Something makes Harry suspect one of his friends is being cheated on. He investigates further and finds out that the alleged cheater spends a lot of time with Severus.
Severus is a jeweller. Harry’s a customer or a detective/an auror investigating a robbery.
Teenage/adult Harry finds an old Muggle porn magazine/video. Severus is in it.
Snooker Snarry: Harry and Severus dressed as player and referee, or partially dressed, or just wearing a bow tie (Harry) and white gloves (Severus). Spanking with a cue, fun with snooker balls, sex on a snooker table etc.
Severus is Albus Severus’ godfather and in love with Harry.
Snarry on “The Love Boat”.
Berry picking.
Severus and Harry play a magical relationship board game for adults.
Severus takes a bus tour. Harry is the guide/another traveller.